Thermoplastic polyolefin (TPO) roofing membranes may be a single layer or may be composed of multiple layers and may contain a reinforcing fabric or scrim reinforcement material in the center between two layers of TPO membrane. A single layer of the TPO roofing membrane must exhibit weatherability (durability), flexibility, longevity, flame retardance, UV resistance, and chemical resistance. In addition, TPO roofing membrane must be capable of forming hot-air welded seams. TPO roofing membrane typically has a thickness from 35-90 mils, with the thicknesses of 45 mil, 60, mil, and 80 mil common industry standards.
Conventional TPO roofing membranes are typically produced in a direct extrusion process on co-rotating twin screw extrusion lines. In direct extrusion, the starting materials are directly fed into an extruder, so that the melting, mixing and extrusion occur simultaneously. Co-rotating twin screw extrusion lines provide high throughput combined with mixing that is capable of mixing high loads of granular flame retardant.
Despite the growing attraction for TPO roofing membranes, polyvinyl chloride (PVC) roofing membranes still occupy a significant portion of the roofing market. In contrast to TPO membrane production, PVC roofing membrane is typically produced on counter-rotating twin screw extrusion equipment which is suitable for the processing of polymers characterized by low thermal stability (i.e., PVC).
Counter-rotating twin screw extrusion has its shortcomings when a high-load of filler is involved. Counter-rotating twin screw extrusion does not allow for even dispersion of high-load flame retardant in conventional TPO polymer systems. Pre-compounded starting material for TPO roofing membrane may be run on conventional PVC extrusion lines. However, the additional process step of compounding increases production and labor expenditures making counter-rotation twin screw extrusion a disadvantaged option compared to co-rotating twin screw extrusion. Consequently, manufacturers of PVC roofing membrane are unable to produce TPO roofing membrane on incumbent counter-rotating twin screw extrusion lines.
The art recognizes the need for a process capable of producing TPO roofing membrane on conventional counter-rotating twin screw extrusion platforms. A need further exists for a process to produce TPO roofing membrane without a compounding step and on a counter-rotating twin screw extruder system.